Conventional wisdom holds that bitmap indexes are most appropriate for columns having low distinct values—such as GENDER, MARITAL_STATUS, and RELATION. This assumption is not completely accurate, however. In reality, a bitmap index is always advisable for systems in which data is not frequently updated by many concurrent systems. In fact, as I'll demonstrate here, a bitmap index on a column with 100-percent unique values (a column candidate for primary key) is as efficient as a B-tree index.

There are several disadvantages to using a bitmap index on a unique column—one being the need for sufficient space (and Oracle does not recommend it). However, the size of the bitmap index depends on the cardinality of the column on which it is created as well as the data distribution. Consequently, a bitmap index on the GENDER column will be smaller than a B-tree index on the same column. In contrast, a bitmap index on EMPNO (a candidate for primary key) will be much larger than a B-tree index on this column. But because fewer users access decision-support systems (DSS) systems than would access transaction-processing (OLTP) ones, resources are not a problem for these applications